Polybutylene terephthalate as typical engineering plastic among thermoplastic polyester resins has been extensively used as a raw material of injection-molded products such as automobile parts, electric and electronic parts and precision equipment parts because of easiness of molding as well as excellent mechanical properties, heat resistance, chemical resistance, aroma-retention property and other physical and chemical properties. In recent years, there is a tendency that polybutylene terephthalate is also used in more extensive applications such as films, sheets, monofilaments and fibers owing to the above excellent properties.
The polybutadiene terephthalate has been usually produced from terephthalic acid or dimethyl terephthalate and 1,4-butanediol as raw materials. In recent years, from the standpoints of high efficiency of use of raw materials, facilitated recovering treatment of by-produced low-molecular weight substances and good availability of the raw materials, there has been employed a so-called direct polymerization method using terephthalic acid and 1,4-butanediol as the raw materials. In particular, from the standpoints of stable quality of products, miniaturization in size of production facilities and good energy efficiency, there has been noticed a direct continuous polymerization method, in which these raw materials are continuously supplied to continuously obtain the products.
In the direct polymerization method, in general, after terephthalic acid is esterified with an excess amount of 1,4-butanediol to obtain an oligomer, the oligomer is polymerized through a polycondensation reaction stage while removing the excessive 1,4-butanediol incorporated into a molecule of the oligomer as well as water produced in the condensation reaction under reduced pressure, thereby producing polybutylene terephthalate. In many cases, in order to accelerate the esterification reaction at an early stage of the process and prevent generation of tetrahydrofuran by side reactions, a titanium compound has been used as a catalyst (for example, Japanese Patent Application Laid-Open (KOKAI) No. 48-47594 (1973)).
However, it is known that the titanium compound tends suffer from problems such as partial deactivation thereof in the course of the esterification reaction and production of impurities, resulting in poor transparency of the obtained products (for example, Japanese Patent Application Laid-Open (KOKAI) No. 2002-284868). The problems tend to become more remarkable in the case of a continuous method (for example, Japanese Patent Application Laid-Open (KOKAI) No. 2002-284870).
In addition, since the titanium compound deactivated in the esterification reaction stage still remains even in the subsequent polycondensation stage, in order to produce polybutylene terephthalate having a desired molecular weight, it is inevitably required to strictly control the polycondensation reaction conditions by raising the reaction temperature or reducing the reaction pressure. However, the use of the high temperature leads to poor color tone of the obtained polymer as well as increase in end carboxyl group concentration thereof, whereas the use of the low pressure requires a very excessive facility capable of achieving the low pressure. Meanwhile, a large amount of the titanium compound is used in order to replenish the deactivated titanium compound, and as a result, the amount of the titanium compound deactivated is correspondingly increased, so that the above problems rather tend to be further accelerated.
On the other hand, in the esterification reaction stage of the direct polymerization method, in order to avoid loss of the raw 1,4-butanediol, it is important to effectively prevent the side reaction in which tetrahydrofuran is produced from 1,4-butanediol. The reaction in which tetrahydrofuran is produced from 1,4-butanediol, tends to be readily caused in the presence of an acid catalyst. Therefore, in the direct polymerization method in which the terephthalic acid is always present in the reaction system, it is very important to increase the velocity of the esterification reaction between terephthalic acid and 1,4-butanediol as a forward reaction. As a useful method for solving this problem, there has been proposed a method of adding a titanium compound to the esterification reaction system (for example, “Journal of Japan Institute of Fibers”, vol. 43, No. 1, p. 35). Meanwhile, in this case, it is considered that the by-production of tetrahydrofuran can be prevented because the titanium compound added exhibits not the effect of preventing conversion of 1,4-butanediol into tetrahydrofuran, but the effect of accelerating the forward reaction.
Also, since the velocity of the side reaction of by-producing tetrahydrofuran depends upon the concentration of 1,4-butanediol, there has been proposed a method of deceasing the amount of the by-produced tetrahydrofuran by decreasing the concentration of 1,4-butanediol in the esterification reaction system (for example, Japanese Patent Application Laid-Open (KOKAI) Nos. 2002-284868 and 2002-284870).
However, the decreased concentration of 1,4-butanediol in the esterification reaction system also tends to cause lowering in velocity of the esterification reaction between terephthalic acid and 1,4-butanediol as the forward reaction. As described above, the side reaction of producing tetrahydrofuran from 1,4-butanediol tends to be readily caused in the presence of an acid catalyst. As a result, even though it is attempted to conduct the forward esterification reaction until reaching a desired conversion rate (esterification conversion), the amount of the by-produced tetrahydrofuran cannot be reduced as desired. Further, the resultant product tends to still exhibit a high solution haze due to deactivation of the catalyst.
On the other hand, in the esterification reaction, it has also been attempted to accelerate the forward reaction by using the titanium compound at a high concentration in order to decrease the amount of the by-produced tetrahydrofuran while maintaining a relatively high concentration of 1,4-butanediol (for example, Japanese Patent Application Laid-Open (KOKAI) No. 62-195017 (1987)).
Meanwhile, it is considered that the deactivation of the titanium compound is caused by transformation of the titanium compound into a higher-molecular weight compound thereof by the effect of water or terephthalic acid. Since the deactivation reaction tends to be accelerated by increased concentration of the titanium compound, in the case where the titanium compound is used at a high concentration as proposed in the above conventional method, the deactivation reaction thereof tends to be inevitably caused, thereby still failing to solve the above problems such as production of impurities and poor transparency. In addition, in such a conventional method, since the catalyst is unremoved in production of the polybutylene terephthalate, a whole amount of the titanium compound used in the esterification reaction is mixed in the obtained polybutylene terephthalate product, thereby inevitably causing problems such as discoloration and poor heat stability of the resultant product due to catalyst residues.
Further, there have been proposed a method of controlling the amount of an organotitanium compound added upon production of the polybutylene terephthalate, and allowing an organotin compound to co-exist in the early esterification reaction stage (for example, Japanese Patent Application Laid-Open (KOKAI) Nos. 2002-284868 and 10-330469 (1998)), and a method of decreasing impurities or haze due to the catalyst by dividing the esterification reaction of continuously reacting terephthalic acid with 1,4-butanediol into two stages, wherein the organotin compound is supplied only to the first esterification reaction stage, and the organotitanium compound is further supplied to the second esterification reaction stage (for example, Japanese Patent Application Laid-Open (KOKAI) No. 10-330468 (1998)).
However, the above conventional methods still fail to solve the problems concerning impurities and haze, and rather have such a problem that the addition of the organotin compound tends to cause deterioration in color tone of the obtained polybutylene terephthalate.
As described above, in any of the conventional direct continuous polymerization methods for producing the polybutylene terephthalate, it has been difficult to prevent deactivation of the titanium compound and simultaneously decreasing the amount of the by-produced tetrahydrofuran in the early esterification reaction stage.